Method of secondary recovery of hydrocarbons

ABSTRACT

A method of producing hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well. It includes the steps of injecting a mobilizing fluid such as steam through the injection well and into the formation. This injection step is continued at a predetermined pressure and for a time interval sufficient to cause breakthrough of the mobilizing fluid to the production well. Thereafter, the pressure level in the production interval is increased and hydrocarbons are produced from the formation while maintaining increased pressure on the formation. The increase in pressure on the producing formation subsequent to breakthrough is maintained by restricting the outflow of producing fluids through the production well while continuing injection of the mobilizing fluid and/or by injecting another fluid down the casing of the producing well while continuing injection of the mobilizing fluid. The injection of the mobilizing fluid at the increased pressure may also be continued until breakthrough occurs to another production well penetrating the formation, with the increased pressure applied to the formation by the aforesaid steps increasing the channeling effect and, hence, improving the effectiveness of the operation.

United States Patent 1191 McBean 1 Nov. 13, 1973 [54] METHOD OF SECONDARY RECOVERY OF Primary ExaminerStephen J. Novosad HYDROCARBONS Attorney-Lee R. Larkin et al.

75] Inventor: William N. McBea B k field, Calif; a 57 ABSTRACT l A method of producing hydrocarbons from a subterra- [73] Asslgnee' Tenn) 9'' Company Houston nean formation penetrated by an injection well and at [22] Filed: May 19, 1972 least one production well. It includes the steps of injecting a mobilizing fluid such as steam through the [21] Appl' 255043 injection well and into the formation. This injection step is continued at a predetermined pressure and for a time interval sufficient to cause breakthrough of the 12211313211511? 191113;:21521511311222: 51 1m. (:1 E2lb 43/22, E2lb 43/24 2 h dmcarbons reduced from the formation 53 Field of Search 166/245, 256, 251, while {naintai in incrpas d fess e on the forma 166/252, 268, 272, 261, 263, 273-275 n e e p r tron. The increase In pressure on the producing formation subsequent to breakthrough is maintained by restricting the outflow of producing fluids through the [56] References Cited production well while continuing injection of the mo- UNITED STATES PATENTS bilizing fluid and/or by injecting another fluid down 2,390,770 12/1945 Barton et al. 166/251 F casing of fi f Y whil? 3,434,541 3/1969 Cook et aL 166/256 ectlon of the mobilizing fluid. The in ection of the 3,459,2 5 3 19 9 Buxton et aL 45 X mobilizing fluid at the increased pressure may also be 3,115,928 12/1963 Campion et a1. 166/259 continued until breakthrough occurs to another pro- 3,199,587 8/1965 Santourian 166/245 duction well penetrating the formation, with the in- 3,208,516 9/1965 Prats l66/245 creased pressure applied to the formation by the g; g i aforesaid steps increasing the channeling effect and, ar er 3,565,173 2/1971 Anderson 166/252 hence mpmvmg the effect'vepess of the operanon 3,608,637 9/1971 Lumpkin et al. 166/256 4 Claims, 1 Drawing Figure METHOD OF SECONDARY RECOVERY OF HYDROCARBONS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of producing hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well. More particularly, it relates to a method of maintaining pressure in a formation while applying a mobilizing fluid thereto such that. the mobilizing fluid will be more efflcient in accomplishing the purpose of the secondary recovery operation, including greater channeling and greater efficiency of the mobilizing fluid.

2. Description of the Prior Art It has become customary in secondary hydrocarbon recovery operations by method of hot fluid (especially steam) injection to allow channeling to occur between the injection well and the producing well and so establish a larger area for heat transfer to occur between the injected fluid and the reservoir rock and fluids, which is deemed necessary for recovery of the hydrocarbons. However, there is a distinct need for better utilization of this channeling phenomenon.

The method of promoting hot fluid breakthroughs to occur results in decreased operating efficiency and can result in decreased recovery if allowed to continue. In a steam displacement operation, for example, steam that is allowed to break through to and vent through a casing bleed represents a heat loss. Also, hydrocarbon vapors carried with the steam phase are generally of a higher economic value than the heavier nonvaporized oil which borders a channel formed through the formation and so represent a loss in reservoir hydrocarbon value unless they are captured at the surface. Also, the heavy oil left behind is generally more viscous and of higher specific heat and is therefore more difficult to recover.

Steam channeling with subsequent breakthrough from the injected well to the producing well or wells is a common occurrence, as stated above. Most-operators have found that breakthrough of this kind is accompanied by an increase inproduction of oil, at least temporarily. It is presently thought'by the Applicant herein that this is primarily due to increased heat transfer area between the live steam and formation rock and fluids and, hence, greater viscosity reduction and fluid swelling benefits are observed. However, these advantages of increased initial production are offset by a number of disadvantages if the channeling is allowed to continue.

Certain disadvantages of this old prior art method include the following:

1. Breakthrough represents a decreased thermal efficiency, as every BTU carried out as sensible heat by the steam and hydrocarbon vapor mixture is lost out the casing vent.

2. Vapor entering the wellbore provides additional production problems in terms of gas locking in the pump. Gas anchors on tubing strings are usually effective, butsometimes holes cut in the gas anchor plug off and gas may be brought into the pump on the suction stroke.

3. The light ends of the native hydrocarbon mixture tend to seek an equilibrium content (in the manner of a steam distillation process) in the flowing vapor stream. The resulting hydrocarbon vapor loss to the gas channel represents an economic loss (since they are generally higher in value) and a recovery efficiency loss (surface recovery is normally not considered economical because of high costs of separation, cooling, etc.). Also, light hydrocarbons which are pushed along through the cold and hot water portions of the flood front have solvent capabilities which are lost if allowed to vent through the casing.

As an example of the prior art, a typical five-spot flood pattern might be utilized wherein there are four production wells located on the four corners of a square, which production wells penetrate the producing formation. There will also be provided an injection well, as for example a steam injection well, in a central position in the square, also penetrating the production formation. By procedures'of the prior art, steam would be injected continuously into the injection well for some period of time and until breakthrough occurs into one of the production wells. If it is determined that the breakthrough is excessive and excess fluids are being produced, the operator, following the prior art teachings, would cut back injection rates to allow some steam to continue through to the production well but at rates which are lower. The effect is to decrease the heat loss through the producing well vent, but this is accomplished at the expense of decreasing the pressure differential between the injecting well and the producing well and at the expense of alower heat transfer rate due to a lowering of the temperature differential between the saturated steam temperature (at a lower pressure) and the formation temperature.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide an improved process of secondary recovery to increase recovery efflciency of the flooding operation in terms of total hydrocarbons recovered and in terms of operating cost efficiency of the secondary recovery operation. It is a further object of this invention to make better use of the channeling phenomenon which occurs in breakthrough from an injection well to a producing well in secondary recovery operations. It is a still furtherobject of this invention to utilize this phenomenon of channeling which occurs during breakthrough and to minimize the disadvantages of such prior art processes.

Briefly stated, this invention is a method of producing hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well. It includes the steps of injecting a mobilizing fluid through the injection well and into the formation. Preferably, the pressure of the injection fluid is at a predetermined pressure differential over the pressure of the producing formation interval adjacent to the production well. The injection step is continued at a pressure level and for a time interval sufficient to cause breakthrough of the mobilizing fluid through the formation to the production well, or one of the production wells in a spot pattern. Subsequently, the pressure in the production interval adjacent the production well to which channeling and breakthrough have occurred is increased and hydrocarbons are produced from the formation while maintaining the increased pressure in the formation. The increase in formation pressure can be accomplished by restricting outflow of produced fluids through the production well while continuing injection of the fluid into the formation. A pressure increase may also be caused by injecting another fluid down the easing of the producing well while continuing injection of the mobilizing fluid into the formation. The mobilizing fluid of this invention may be any one of a number of fluids including a heated fluid such as steam, vaporizing fluids, fluid solvents, combustion fluids, surfactant fluids, combinations thereof, and the like.

Once a breakthrough to one producing well has been accomplished, the injection of mobilizing fluid at the increased pressure is continued until breakthrough occurs to another production well in the support pattern. When breakthrough occurs at the second production well, the pressure in the production interval adjacent thereto can likewise be increased as with the initial production well.

The invention may be further understood by reference to the drawing and to the description provided hereinafter.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows a typical nine-spot pattern of an injection well and a plurality of production wells in a hypothetical 9.3 acre are which will be utilized to des r b %B l @l2l efthsiil q t DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the prior art five-spot pattern described above, the present invention would be carried out by partially shutting in the casing vent on the production well to which breakthrough first occurred. The immediate effect would be to decrease the well productivity. However, it would also promote channeling of the steam into other sinkholes in the well pattern, as for example the other production wells. Steam and hydrocarbon vapors in the channel between the injected well and the first production well would tend to condense. However, if some small amount of steam is continuously let off the initial production well, the channel will remain open and allow a continued larger area heat transfer. By increasing injection pressures, the pressure differential between the injection well and the first production well will be maintained. In addition, the flood front will continue to advance and the initial production well productivity will be maintained. At the same time, the now heightened differential pressure between the injector well and the other production wells will promote greater channeling tendencies to these wells. As breakthrough occurs at these wells, their formation pressures are increased in the manner with respect to the first production well. Eventually, all producers will have increased back pressure and the flood pattern may be assumed to have been established. Injection pressures and producing pressures would then be chosen to maintain a sufficient differential for flood advance between the two. It is contemplated that sufficient back pressure will be applied to the producing well formation to insure that little or no hydrocarbon vapors are allowed to blow through to the casing vent of the production well or wells. Steam may be continuously let off at very low rates in order to keep the channel free for optimum heat transfer.

There are a number of advantages to this invention of repressuring the injector and/or producer well to elevated pressures and they include the following:

I. There is increased solvent action of condensable hydrocarbons in the channel at the given temperatures and pressures.

2. A larger area of heat transfer is maintained but not at the expense of losing high economically valued constituents or components of the native crude.

3. The elevated pressure allows for increased temperature differentials between injected steam and the formation, assuming that the quality steam at the given pressures is maintained. This results in greater heat transfer by conduction and radiation.

4. The heat content of steam injected at elevated temperatures is higher per unit volume and, hence, more heat may be injected with less tendency to adversely affect the mobility ratios due to over-saturation of the formation with water.

5. Repressuring of the producer wells and the injector wellselectively as breakthrough occurs tends to promote fasterdevelopment of the full flood pattern as the steam, or other mobilizing fluid, is forced to break through to the lower pressure portions of the pattern that have not evidenced breakthrough.

6. Somewhat higher gravity crude will be produced from the flood pattern and, therefore, higher revenues realized.

7. There will be less tendency to leave the heavy, viscous, high specific heat oil residue bordering the steam channel or fluid channel which would, under lower pressures and temperatures associated with the prior art, become virtually unrecoverable.

There are certain limitations to the operation of the present invention which will need to be considered in carrying out the method in any particular formation, and the pressures required therefore. For example, the surface equipment must be sufficient to maintain steam quality and, hence, surface generation equipment must be capable of producing high quality steam at elevated pressures. Further, well completions, as for example wellhead structures, must be capable of withstanding thermal expansion at elevated temperatures. Fluid transporting equipment must also be capable of withstanding elevated temperatures. For most applications, the injection pressure will have to be kept below the fracture gradient of the formation to assure good sweep efficiency. The elevated pressures contemplated by this invention may tend to limit fluid swelling at elevated temperatures and may sufficiently affect this steam displacement recovery mechanism to limit absolute pressure levels. This is likely to be most severe when steam flooding of light oil formations is contemplated.

Nevertheless, the design or purpose of this invention is to decrease the vaporization of the light components of the hydrocarbons by increasing superimposed pressure on the hydrocarbon bearing formation at the injection and/or production wells. The process may be extended to cover other fluid injection where the effect of the injected fluid is to increase the mobility of a portion of the total hydrocarbons, which is not advantageous to total ultimate recovery (either by miscibility with the injected fluid or by change of fluid flow characteristics). The process calls for an increase in the superimposed pressure against a producing interval after breakthrough of the injected fluid has occurred. Similarly, the injection pressure will be increased to obtain whatever differential between injector well and producer well is deemed necessary. The increased pressure against a producing interval may be accomplished by shutting the casing (either completely or partially) or by injection of another fluid down the casing annulus (a diluent, inert gas, casing gas, etc.). The increased back pressure against any portion of the flood pattern will promote breakthrough to other low pressure portions of the flood pattern. When breakthrough has occurred at all producing wells, injection pressures and producing well pressures may be set at whatever absolute pressure readingsare devised to maintain the optimum pressure differential. Timing of the repressuring of producer wells or injector wells is to be determined by the effects of repressuring on the complete operation, including other flood patterns. Referring now to the drawing, an illustrative example will be given showing operation of the present invention.

The drawing shows a typical regular nine-spot pattern having an injection well I1 and production wells Pl through P8. By way of illustration, the injection and production wells may extend to a shallow heavy oil reservoir on the order of 1,500 feet in depth, where a steam flood is to be carried out. The hydrocarbon properties might, for example, be as follows:

1. API

2. ASTM lnitial Boiling Point 293F 3. Cracking beyond 700F 4. Viscosity 188 cp at 60F 8 cp at l80F In carrying out the invention, a mobilizing fluid such as steam is injected into injector well [1 at an injection pressure corrected to bottomhole formation elevation at approximately 400 psig, for example. Assuming that saturated steam at 70 percent quality is exiting from the injection well ll wellbore to the formation, the flooding is ready for commencement. It is also assumed that the formation pressure in the production interval of the production wells is zero, that is, all of the production wells are pumped off to a point below the producing interval.

Therefore, at this stage of the process there exists a 400 psi pressure differential superimposed on the formation between the injector well I1 and the production wells Pl through P8.

The injection of the steam is continued until breakthrough occurs to one of the production wells, as for example P4. There is thus provided between injector well I1 and production well P4 a channeling. Accompanying this steam'breakthrough there will normally be an increase in the hydrocarbon production from P4. In addition to the increased oil recovery rate, P4 will also normally be venting some amounts of volatile hydrocarbons which are carried off with the steam in a manner of steam distillation process unless this venting is stopped immediately after breakthrough. Any continued venting of this nature represents a heat loss, for instance, as associated with the heat content of the steam and hydrocarbons, and an economic loss (associated with the sale value of the hydrocarbons carried with the steam).

If prior art methods were to be followed, the steam injection rate into I1 would be cut back to some rate such that the steam channeling to P4 would not be excessive. There are many disadvantages to this prior art step including the reduced injection pressures assoresult in decreased project sales yield due to the time dependency of the hydrocarbon sales value (per the present value concept of future recoverable monies). In addition, the decrease in injected fluid temperature thermodynamically defined by the injection pressure results in a decrease in the rate of heat transfer from injected fluid to the formation rock and fluids by conductive and radiant heat transfer mechanisms. These heat transfer mechanisms are functions of the temperature differentials between the hot and cold masses (in this case, steam and formation rock and fluids). Since this heat transfer is the basic goal in injecting a heated fluid, any reduction in the transfer rate must be considered an adverse effect.

In carrying out the invention herein, the production well P4 is subjected to back pressure against the producing interval and the injection rate in II is raised such that the pressure differential between I1 and P4 is restored to the original level, i.e. 400 psi. This back pressuring of P4 may be accomplished by throttling the casing vent valve of P4 or by injecting another fluid down the casing annulus, which fluid might be a diluent, inert gas, steam, casing-head gas, or the like.

Following this method, the producing rate of P4 is restored due to the fact that the pressure differential has been restored. Also, there now exists an even greater differential between injector well I] and the other producing wells, thereby promoting faster breakthrough to these wells. As breakthrough occurs at each of the other wells, repressuring is repeated thereat as described above with respect to well I].

It is preferred that the back pressure on P4 and injection pressure levels at [1 are chosen such that some small amount of steam is continuing through the fluid channel between the two wells. in this way, the advantageous effects of the breakthrough phenomena are still utilized.

The immediate advantageous effects of the increased pressure levels are as follows:

1. There is promotion of faster breakthrough to other producing wells due to the increased pressure differential.

2. Higher heat transfer rates from injected fluid to formation rock and fluids are provided due to the increased temperature differential.

3. There is greater utilization of the solvent capabilities of the light hydrocarbons in the channel due to condensation at elevated pressures.

4. Injected fluid at the elevated pressure will carry more heat per unit mass which becomes available for the desired heat transferred to the formation.

5. There is less tendency to leave a. heavy viscous high specific heat oil residue bordering the channel, which would, under lower temperatures and pressures, become virtually unrecoverable. V

6. A large area for heat transfer is maintained, but not at the expense of losing the high economic value components of the native crude.

A flood pattern development may thus be accelerated and somewhat controlled by sequentially backpressuring production wells which have experienced breakthrough and increased injection pressures.-

Mobilizing fluids may be of various types and properties and include the following:

1. Heated fluids including any injected fluid which is heated externally of the formation and includes heated water or steam, inert gas (including nitrogen, helium and the like), flue gases from internal combustion engines, carbon dioxide, carbon monoxide, and the like.

2. Vaporizing fluids including any injected fluid which causes a reduction in the vaporization temperature of the formation hydrocarbon mixture components. Examples of such vaporizing fluids includes steam or any chemically inert gas that has the effect of lowering the partial pressures of the hydrocarbon components in the formation.

3. Fluid solvents including any materials that, when mixed with the hydrocarbon mixture, effect a lower bulk viscosity of the composite mixture. Examples of such fluid solvents include benzene, toluene, normal and iso-alkane hydrocarbons of low molecular weight (C-l through C-lS).

Combustion fluids including those fluids injected to establish an insitu combustion in the formation and to include the products of the combustion reaction. Examples of combustion fluids are air, natural gas, carbon monoxide, carbon dioxide, nitrous oxide, steam, and the like.

Surfactant fluids include those fluids which change the surface tension characteristics of the formation fluids, as for example detergents.

It is to be understood that the term mobilizing fluid, as used herein, is to include but is not necessarily limited to all of the foregoing fluids and combinations thereof.

There are several advantages associated with utilization of the present invention including initial high oil recovery rates and increased heat transfer area made available by the occurrence of a fluid channel, as for example a hot fluid channel. (The conductive rating heat transfer mechanisms functions are varried.) It is the intention of this invention to utilize the advantageous effects of the hot fluid injection and minimize the adverse effects of the breakthrough phenomenon.

Moreover, it should be understood that while the foregoing specific embodiment has been described with respect to steam displacement operation, the invention is not limited to steam as the injection fluid. Several types of gas or hot fluid injection will promote an elevation of the component hydrocarbon vapor pressures or may alter the immiscibility of hydrocarbons with the injected phase or may alter the mobility of portions of the native hydrocarbon mixture that would adversely affect the total recovery or the operating efficiency of the displacement process. Control of production well backpressure in an insitu combustion operation will enable better control of flood advance and the products of combustion would at least partially be re-established in solution for viscosity reduction benefits by the increased producer backpressure. The process of increasing the absolute pressure levels while maintaining the necessary differential is not limited to one type of flood pattern nor to any single type of fluid injection.

There are many other secondary flooding operations which will benefit from the invention herein. Whenever the injected fluid tends to flood past the formation leaving heavy, viscous residual oil, repressuring according to this invention may beneficially affect the miscibility or mobility of the combined formation hydrocarbon mixture. For example, in an insitu combustion process, the present invention will result in increasing the miscibility of the product of combustion with the oil head at the firefront. Any other gas or fluid injection which forms asiotrophic or other mixtures which tend to increase the relative vapor pressures of the hydrocarbon components for a given temperature would benefit by the teachings of this invention.

The disclosure then is not intended to cover any single flood pattern or single type of fluid injection. It is meant to describe a production technique which has particular suitabilities to a plurality of secondary steam flood recoveries, or other flood recoveries where bydrocarbon vaporization, miscibility or mobility may be controlled beneficially by the repressuring'steps of this invention.

Further modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the invention. It is to be understood that the form of the invention herewith shown and described is to be taken as the presently preferred embodiment. Various changes may be made in the shape, size and arrangement of parts. For example, equivalent elements or materials may be substituted for those illustrated and described herein, parts may be reversed, and certain features of the invention may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the invention.

What is claimed is:

1. In a method of producing hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well, the combination of steps comprising:

injecting a mobilizing fluid through said injection well and into said formation;

continuing said injecting step at a pressure level and for a time interval sufficient to cause breakthrough of said mobilizing fluid through said formation to said at least one production well;

subsequently increasing the pressure in the production interval in said formation adjacent said production well after said breakthrough by injecting another fluid down said productionwell while continuing injection of mobilizing fluid into said formation;

and producing hydrocarbon from said formation while maintaining said increased pressure in said formation.

2. In a method of producing hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well, the combination of steps comprising:

injecting a mobilizing fluid through said injection well and .into said formation;

continuing said injecting step at a pressure level and for a time interval sufficient to cause breakthrough of said mobilizing fluid through said formation to said at least one production well;

subsequently increasing the pressure in the production interval in said formation adjacent said production well after said breakthrough by restricting outflow of produced fluids through said production well and by injecting another fluid down said production well while continuing injection of said mobilizing fluid into said formation;

and producing hydrocarbon from said formation while maintaining said increased pressure in said formation.

3. in a method of secondary recovery of hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well, the combination of steps comprising:

injecting a mobilizing fluid through said injection well and into said formation at a predetermined pressure differential over the pressure of the production formation interval adjacent said one production well; continuing said injecting step at said pressure differential and for a time interval sufficient to cause breakthrough of said mobilizing fluid through said formation to said one production well, thereby causing a drop in said pressure differential;

subsequently increasing the pressure in said production interval to a higher pressure level higher than that which existed immediately after said breakthrough by injecting another fluid down said one production well into a portion of said production interval;

' continuing the injection of mobilizing fluid until breakthrough occurs to another one of said production wells;

subsequently increasing the pressure in the production interval adjacent said other production well to a higher pressure level than that which existed immediately after said breakthrough thereto;

and producing hydrocarbons from said formation while maintaining said increased pressure on said production interval.

4. The invention as claimed in claim 3 wherein:

the continued injection of mobilizing fluid subsequent to said breakthroughs is at a pressure level sufficiently high to maintain a pressure differential between said mobilizing fluid and said production interval at least as high as the pressure differential therebetween that existed prior to said breakthroughs. 

1. In a method of producing hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well, the combination of steps comprising: injecting a mobilizing fluid through said injection well and into said formation; continuing said injecting step at a pressure level and for a time interval sufficient to cause breakthrough of said mobilizing fluid through said formation to said at least one production well; subsequently iNcreasing the pressure in the production interval in said formation adjacent said production well after said breakthrough by injecting another fluid down said production well while continuing injection of mobilizing fluid into said formation; and producing hydrocarbon from said formation while maintaining said increased pressure in said formation.
 2. In a method of producing hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well, the combination of steps comprising: injecting a mobilizing fluid through said injection well and into said formation; continuing said injecting step at a pressure level and for a time interval sufficient to cause breakthrough of said mobilizing fluid through said formation to said at least one production well; subsequently increasing the pressure in the production interval in said formation adjacent said production well after said breakthrough by restricting outflow of produced fluids through said production well and by injecting another fluid down said production well while continuing injection of said mobilizing fluid into said formation; and producing hydrocarbon from said formation while maintaining said increased pressure in said formation.
 3. In a method of secondary recovery of hydrocarbons from a subterranean formation penetrated by an injection well and at least one production well, the combination of steps comprising: injecting a mobilizing fluid through said injection well and into said formation at a predetermined pressure differential over the pressure of the production formation interval adjacent said one production well; continuing said injecting step at said pressure differential and for a time interval sufficient to cause breakthrough of said mobilizing fluid through said formation to said one production well, thereby causing a drop in said pressure differential; subsequently increasing the pressure in said production interval to a higher pressure level higher than that which existed immediately after said breakthrough by injecting another fluid down said one production well into a portion of said production interval; continuing the injection of mobilizing fluid until breakthrough occurs to another one of said production wells; subsequently increasing the pressure in the production interval adjacent said other production well to a higher pressure level than that which existed immediately after said breakthrough thereto; and producing hydrocarbons from said formation while maintaining said increased pressure on said production interval.
 4. The invention as claimed in claim 3 wherein: the continued injection of mobilizing fluid subsequent to said breakthroughs is at a pressure level sufficiently high to maintain a pressure differential between said mobilizing fluid and said production interval at least as high as the pressure differential therebetween that existed prior to said breakthroughs. 